A sound reproducing apparatus using an in-ear earphone is compact, highly portable, and useful. On the other hand, since wearing an earphone in an ear blocks an ear canal, there arises a problem that the sound is slightly muffled and that it is difficult to obtain a spacious sound.
For example, let it be assumed that the ear canal of an ear is represented by a simple cylindrical model. When not wearing an earphone in the ear, the cylinder is closed at the eardrum side and is open at the entrance side of the ear, that is, one end of the cylinder is open and the other end is closed ((a) in FIG. 16). In this case, a primary resonance frequency is about 3400 Hz if it is assumed that the length of the cylinder is 25 mm which is an average length of the ear canal of a human. On the other hand, when wearing an earphone 110 in the ear, the cylinder is closed at the eardrum side and the entrance side of the ear, that is, both ends of the cylinder are closed ((b) in FIG. 16). In this case, a primary resonance frequency is about 6800 Hz which is double that in the case of not wearing an earphone.
One of techniques to solve the above problem is a conventional sound reproducing apparatus which corrects a resonance frequency characteristic of an ear canal to reproduce a sound, thereby realizing a listening state equivalent to that in the case of not wearing an earphone (in the case where the ear canal is not blocked), even when, actually, wearing the earphone in the ear (for example, see Patent Document 1).
FIG. 17 shows a configuration of a conventional sound reproducing apparatus 1700 disclosed in Patent Document 1. In the conventional sound reproducing apparatus 1700 shown in FIG. 17, a correction information storage section 1703 stores correction information about an ear-canal impulse response variation, and a convolution operation section 1704 convolves a sound source signal with the correction information, thereby realizing a listening state equivalent to that in the case where the ear canal is not blocked.
Moreover, there is a conventional acoustic-field reproducing apparatus which automatically measures a head-related transfer function of a listening person with use of an in-ear transducer used for both a microphone and an earphone, and convolves an inputted signal with the measured head-related transfer function of the listening person, and which allows the listening person to receive the convolved signal via the in-ear transducer used for both a microphone and an earphone (for example, see Patent Document 2). The conventional acoustic-field reproducing apparatus realizes, through the above processing, the effect of allowing an unspecified listening person to obtain excellent feeling of localization of a plurality of sound sources present in all directions.    Patent Document 1 Japanese Laid-Open Patent Publication No. 2002-209300    Patent Document 2 Japanese Laid-Open Patent Publication No. H05-199596